In order to operate in a satisfactory manner, an optical imaging system must exhibit good optical qualities, for example as regards the wave surface quality and the stability of the boresight.
When an imaging system exhibits a good wave surface quality, a point in the scene forms, in the image plane, an image dot exhibiting a light intensity distribution with dimensions essentially limited by the intrinsic resolution of the imaging system (diffraction limit). Any defect or incorrect positioning of the optical surfaces which compose the imaging system leads to a wave surface defect which results in an increase in the dimensions of the image dot and a decrease in the resolution of the system.
Generally speaking, a initially correctly adjusted system may exhibit a shift in the quality of its wave surface during the course of its operation.
For imaging systems placed onboard satellites for observing the Earth, such as instruments like high-resolution telescopes, monitoring of the variations in the wave surface is necessary in order to guarantee the quality of the recorded images.
One method for measuring the wave surface is to frequently acquire calibration images in order to follow the variation of the wave surface.
These calibration images may be obtained for example by taking specific photographs of specific external scenes: dedicated ground sites, stars.
Another known solution is to equip the instrument with an auto-calibration system comprising a mobile header mirror which, for the calibration, is oriented toward an internal target.
These solutions have the drawback of requiring a specific programming of the satellite, which may be at a high rate, for example every 20 min, each measurement requiring an interruption of the nominal programming operations of the satellite and potentially a displacement of mobile components.
Thus, the acquisition of calibration images leads to severe operational constraints.
Furthermore, in order to operate in a satisfactory manner, an optical imaging system must exhibit a good boresight stability. This stability is to be considered over the integration time scale in order to avoid the “blurring of the images” during the integration time and over the longer term in order to precisely reproduce the localization of the observed scenes.
These instabilities of the boresight are all the greater the smaller the IFOV (acronym for Instantaneous Field of View). For onboard instruments, the greatest instabilities are vibrational in origin; they determine the mico-vibrational stability performance of the platform. Other instabilities are thermo-elastic in origin; they determine the thermo-elastic stability performance of the imaging system.
The aim of the present invention is to overcome the aforementioned drawbacks.